Some focused ion beam (FIB) columns are intended for use with ion sources which emit multiple ion species. In order to select only one of these ion species for the beam to be focused on a substrate, the FIB column will typically include a mass filter. One type of mass filter, a “Wien filter,” uses crossed electric and magnetic fields (E×B) to deflect unwanted ion species off-axis, thereby causing them to strike a mass-separation aperture and is also referred to as an “E×B filter.” An E×B filter operates according to principles well-known in the art: crossed electric and magnetic fields (both generally perpendicular to the beam direction through the mass filter) induce forces on the ions in the beam in opposite directions transversely to the beam motion. The relative strengths of these two forces are determined by the electric and magnetic field strengths, controlled by voltage and current supplies that energize the electrodes and magnetic poles.
FIG. 1 is a side cross-sectional view of a prior art Wien (E×B) mass filter 102 in a focused ion beam (FIB) column 104 that includes an upper lens 106 and a lower lens 108 that combine to focus an ion beam onto a substrate surface 112. Ions 110 of three different ion species are shown being emitted by a source tip 114 induced by a voltage applied between the source tip 114 and an extractor electrode (not shown). This source structure is typical of that in a liquid metal ion source (LMIS), however other types of ion sources may be used in the prior art. Ions 110 are then focused by the upper lens 106 into a plane 120 of a mass separation aperture 122. The Wien filter 102 includes electrodes 130 that produce an electrostatic field and a source (not shown) of a magnetic field such as a coil or permanent magnet (the magnetic poles would be in front of and behind the plane of FIG. 1). Wien filter 102 deflects low mass ions 136 and high mass ions 138 off-axis, leaving the middle mass ions 140 largely undeflected. These middle mass ions 140 then pass through the aperture 122 and are focused by the lower lens 108 onto substrate surface 112. As shown by the formulas below, low mass ions 136 have higher velocities for the same beam energy than do high mass ions 138. Since the electric force is the same for all ions (having the same charge) while the magnetic force is proportional to velocity, the faster low mass ions 136 will be deflected more by the magnetic field than the slower high mass ions 138—thus the lower mass ions are deflected in the direction of the magnetic force (to the left), while the high mass ions are deflected in the direction of the electric force (to the right). For middle mass ions 140, the electric and magnetic forces are balanced (i.e., having equal magnitudes in opposite directions), giving no net force.
In FIG. 1, an electric field 142 is horizontal in the plane of the figure (pointing from the positive electrode 130 at the left towards the negative electrode 130 at the right (making the electric force on a positive ion towards the right), while the magnetic field 144 is perpendicular to, and pointing out of, the plane of the figure (making the magnetic force on a positive ion towards the left). If the ion source 114 is emitting a number of ion species with different charge-to-mass ratios, it is possible to set the electric field 142 and magnetic field 144 strengths so that one ion species may pass through the E×B mass filter undeflected—in FIG. 1, this species is the middle mass ions 140. The low mass ions 136 and high mass ions 138 are deflected to the left and right, respectively, as shown. Only the middle mass ions 140 pass through the mass separation aperture 122, to then be focused onto the substrate surface 112 by the lower lens 108. At the top and bottom of the E×B mass filter 102, field termination plates 150 cut off both the electric and magnetic fields, thereby reducing fringe-field aberrations.
To better understand the aberrations induced by the E×B filter, FIG. 2 shows the same two-lens column 104 and prior art E×B mass filter 102 as in FIG. 1, but with the mass separation aperture plate 120 (see FIG. 1) removed. Note that ions of the three different species (high, middle and low mass) are focused at three different locations on the substrate due to the deflection of the Wien filter. Ions having the same mass but with differing energies will also be deflected differently by the E×B filter due to chromatic aberration.
With no aperture plate 122, all the ions will pass to substrate surface 112 as shown. The E×B filter deflects the low mass ions 136 to the left and the high mass ions 138 to the right at the plane of the lower lens. The three crossovers (low mass crossover 236, middle mass crossover 240 and high mass crossover 238) formed by the focusing effects of the upper lens 106 combined with the mass separation effects of the E×B mass filter 102 form “virtual sources” that are imaged by the lower lens 108 onto the substrate 112. Because these three virtual sources are spatially separated by the E×B filter, their three respective images at the substrate 112 are also separated as shown—the separation distances at the substrate 112 are demagnified by lens 108 from the corresponding separations of crossovers 236, 240, and 238 at the plane of the (removed) mass separation aperture 120. Similarly, the chromatic aberration of the E×B causes the separation of ions having the same mass but different energies. This can be seen from the (non-relativistic) equation for the ion velocity at the E×B filter:½m·v2=n e V=the energy of the ionv=sqrt(2 n e V/m)=the velocity of the ion
Where                m=the ion mass        v=the ion velocity        n=the ion charge state (1=singly-ionized, 2=doubly-ionized)        e=the elemental charge        V=the accelerating potential in the electron gun        
If the two fields in the E×B filter are:                E=electric field        B=magnetic field        
Then the net force on an ion passing through the filter with a velocity, v, will be:Ftotal=Felectric+Fmagnetic=n·e·[E−(v/c)·B]
Where                Felectric=n·e·E        Fmagnetic=−n·e·(v/c)·B (opposite in direction from Felectric)        
Thus, the Wien filter is seen to be actually a velocity filter. Because ions of differing masses (and the same nominal energies) will have differing velocities (lower masses faster, higher masses slower), Wien filters are commonly used (and referred to) as “mass filters.” Even for a single ion species, however, there will be a spread in velocities due to the inherent spread in energy (around the nominal energy) of ions emitted by any type of ion source—for example, energy spreads from liquid metal ion sources typically have FWHM energy spreads of around 5 eV. The dispersion effects on the beam due to these energy spreads cause a chromatic aberration which will blur the focused beam at the substrate if not corrected.
Another disadvantage of the ion beam column shown in FIG. 1 is that it includes a beam crossover, that is, a point in the beam path where the ions cross the optical axis. The crossover has three deleterious effects: 1) electrostatic repulsions are increased as the particles are brought closer together at the crossover itself, 2) forming a crossover generally makes the beam diameter smaller throughout the entire column (when compared with the beam diameter without a crossover), also increasing space-charge effects, and 3) sputtering of the mass separation aperture is increased at the impact points of the non-transmitted ion beams since these beams are focused at the plane of the aperture (e.g., at crossovers 236 and 238), thereby increasing the beam current densities and thus the sputter rates perpendicular to the plane of the mass separation aperture (i.e., focused non-selected beams sputter through the aperture faster than unfocused non-selected beams).
The electrostatic repulsion at the crossover spreads the beam, reducing the beam current density at the substrate surface. At the crossover, two separate electrostatic repulsion effects occur:
1) Boersch Effect—this is the increase in beam energy spread due to axial beam scattering. Essentially, one ion gains energy at the expense of another.
2) Loeffler Effect—this is the sideways scattering of the charged particles, causing the final focused spot to be larger and/or more blurry.
“Achromatic two-stage E×B mass filter for a focused ion beam column with collimated beam”, Teichert, J., and Tiunov, M. A., Meas. Sci. Technol. 4 (1993) pp. 754-763, described in more detail below, describes a two stage mass filter with reduced chromatic aberration used with a collimated beam.
Another problem with the column shown in FIG. 1 is that unfocused neutral particles can reach substrate surface 112. Ion sources incidentally also emit neutral particles, while other neutral particles may result from gas-ion collisions occurring between the source tip 114 and the mass filter 102. In the column of FIG. 1, some of these neutral particles will pass through aperture 122 and reach the substrate. Because neutral particles do not respond to the fields of the focusing lenses, neutral particles that are not blocked by the aperture plate are spread out over areas of substrate surface 112 typically much larger than the area scanned by the focused ion beam.